Java căn bản - Chapter9


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  • We will cover the basic string processing in this lesson, manipulating char and String data.
  • Characters can be stored in a computer memory using the ASCII encoding. The ASCII codes range from 0 to 127. The character 'A' is represented as 65, for example. The ASCII values from 0 to 32 are called nonprintable control characters. For example, ASCII code 04 eot stands for End of Transmission. We can use this character to signal the end of transmission of data when sending data over a communication line.
  • ASCII works well for English-language documents because all characters and punctuation marks are included in the ASCII codes. But ASCII codes cannot be used to represent character sets of other languages. To overcome this limitation, unicode encoding was proposed. Unicode uses two bytes to represent characters and adopts the same encoding for the first 127 values as the ASCII codes. Encoding value of a character can be accessed by converting it to an int as the sample code illustrates.
  • We introduced the String class in Chapter 2. We will study additional String methods.
  • This sample code counts the number of vowels in a given input. Using the toUpperCase method, that converts all alphabetic characters to uppercase, we can rewrite the code as char letter; String name = inputBox.getString("What is your name?"); int numberOfCharacters = name.length(); int vowelCount = 0; String nameUpper = name.toUpperCase(); for (int i = 0; i < numberOfCharacters; i++) { letter = nameUpper.charAt(i); if ( letter == 'A' || letter == 'E' || letter == 'I' || letter == 'O' || letter == 'U' ) { vowelCount++; } } System.out.print(name + ", your name has " + vowelCount + " vowels");
  • This sample code counts the number of times the word 'Java' is entered. Notice that we wrote word.equals( “STOP”) not word == “STOP” We described the differences in Lesson 5-2. In this situation, we need to use 'equals' because we are testing whether two String objects have the same sequence of characters.
  • Here are some examples: String str1 = “Java”, str2 = “ Wow “; str1.compareTo( “Hello” ); //returns positive integer //because str1 >= “Hello” str1.substring( 1, 4 ); //returns “ava” str2.trim( ) //returns “Wow”, str2 stays same str1.startsWith( “Ja” ); //returns true str1.endsWith( “avi” ); //returns false
  • We can use a pattern to represent a range of valid codes succintly. Without using the sample 3-digit pattern for computer science majors living on-campus, we must spell out 21 separate codes.
  • Regular expression allows us to express a large set of “words” (any sequence of symbols) succinctly. We use specially designated symbols such as the asterisk to formulate regular expressions.
  • Here are some examples of regular expressions.
  • The start method returns the position in the string where the first character of the pattern is found. The end method returns the value 1 more than the position in the string where the last character of the pattern is found.
  • By making String objects immutable, we can treat it much like a primitive data type for efficient processing. If we use the new operator to create a String object, a separate object is created as the top diagram illustrates. If we use a simple assignment as in the bottom diagram, then all literal constants refer to the same object. We can do this because String objects are immutable.
  • If the situation calls for directing changing the contents of a string, then we use the StringBuffer class.
  • This sample code illustrates how the original string is changed. Notice that the String class does not include the setCharAt method. The method is only defined in the mutable StringBuffer class.
  • Notice how the input routine is done. We are reading in a String object and converting it to a StringBuffer object, because we cannot simply assign a String object to a StringBuffer variable. For example, the following code is invalid: StringBuffer strBuffer = inputBox.getString( ); We are required to create a StringBuffer object from a String object as in String str = "Hello"; StringBuffer strBuf = new StringBuffer( str ); You cannot input StringBuffer objects. You have to input String objects and convert them to StringBuffer objects.
  • The append and insert are the two very useful methods of the StringBuffer class for string manipulation.
  • For this application, we are given two helper classes. The FileManager class handles the file input and output. The WordList class handles the maintenance of word lists. Our responsibility is to extract the words from a given document and use the helper classes correctly.
  • There will be a total of six key classes in this application. We will be designing two classes: Ch9WordConcordanceMain and Ch9WordConcordance.
  • Please use your Java IDE to view the source files and run the program.
  • For the second extension, the WordList class itself needs to be modified. Details on how to implement this extension can be found in Chapter 10 of the textbook.
  • Java căn bản - Chapter9

    1. 1. Chapter 9 Characters and Strings
    2. 2. Objectives <ul><li>After you have read and studied this chapter, you should be able to </li></ul><ul><ul><li>Declare and manipulate data of the char data type. </li></ul></ul><ul><ul><li>Write string processing program using String, StringBuilder, and StringBuffer objects. </li></ul></ul><ul><ul><li>Differentiate the three string classes and use the correct class for a given task. </li></ul></ul><ul><ul><li>Specify regular expressions for searching a pattern in a string. </li></ul></ul><ul><ul><li>Use the Pattern and Matcher classes. </li></ul></ul><ul><ul><li>Compare the String objects correctly. </li></ul></ul>
    3. 3. Characters <ul><li>In Java, single characters are represented using the data type char . </li></ul><ul><li>Character constants are written as symbols enclosed in single quotes. </li></ul><ul><li>Characters are stored in a computer memory using some form of encoding. </li></ul><ul><li>ASCII , which stands for American Standard Code for Information Interchange , is one of the document coding schemes widely used today. </li></ul><ul><li>Java uses Unicode , which includes ASCII, for representing char constants. </li></ul>
    4. 4. ASCII Encoding For example, character 'O' is 79 (row value 70 + col value 9 = 79). O 9 70
    5. 5. Unicode Encoding <ul><li>The Unicode Worldwide Character Standard ( Unicode ) supports the interchange, processing, and display of the written texts of diverse languages. </li></ul><ul><li>Java uses the Unicode standard for representing char constants. </li></ul>char ch1 = 'X'; System.out.println(ch1); System.out.println( (int) ch1); X 88
    6. 6. Character Processing Declaration and initialization char ch1, ch2 = ‘X’; Type conversion between int and char. System.out.print(&quot;ASCII code of character X is &quot; + (int) ' X ' ); System.out.print (&quot;Character with ASCII code 88 is &quot; + (char)88 ); This comparison returns true because ASCII value of ' A ' is 65 while that of ' c ' is 99. ‘ A’ < ‘c’
    7. 7. Strings <ul><li>A string is a sequence of characters that is treated as a single value. </li></ul><ul><li>Instances of the String class are used to represent strings in Java. </li></ul><ul><li>We can access individual characters of a string by calling the charAt method of the String object. </li></ul>
    8. 8. Accessing Individual Elements <ul><li>Individual characters in a String accessed with the charAt method. </li></ul>0 1 2 3 4 5 6 S u m a t r a String name = &quot;Sumatra&quot; ; name This variable refers to the whole string. name.charAt( 3 ) The method returns the character at position # 3.
    9. 9. Example: Counting Vowels char letter; String name = JOptionPane.showInputDialog(null, &quot;Your name:&quot; ); int numberOfCharacters = name.length () ; int vowelCount = 0; for ( int i = 0; i < numberOfCharacters; i++ ) { letter = name.charAt ( i ) ; if ( letter == 'a' || letter == 'A' || letter == 'e' || letter == 'E' || letter == 'i' || letter == 'I' || letter == 'o' || letter == 'O' || letter == 'u' || letter == 'U' ) { vowelCount++; } } System.out.print ( name + &quot;, your name has &quot; + vowelCount + &quot; vowels&quot; ) ; Here’s the code to count the number of vowels in the input string.
    10. 10. Example: Counting ‘Java’ Continue reading words and count how many times the word Java occurs in the input, ignoring the case. int javaCount = 0; boolean repeat = true ; String word; while ( repeat ) { word = JOptionPane.showInputDialog ( null , &quot;Next word:&quot; ) ; if ( word.equals ( &quot;STOP&quot; ) ) { repeat = false; } else if ( word.equalsIgnoreCase ( &quot;Java&quot; ) ) { javaCount++; } } Notice how the comparison is done. We are not using the == operator.
    11. 11. Other Useful String Operators <ul><li>See the String class documentation for details. </li></ul>Returns true if a string ends with a specified suffix string. str1.endsWith( str2 ) Returns true if a string starts with a specified prefix string. str1.startsWith( str2 ) Converts a given primitive data value to a string. String.valueOf( 123.4565 ) Removes the leading and trailing spaces. str1.trim( ) Extracts the a substring from a string. str1.substring( 1, 4 ) Compares the two strings. str1.compareTo( str2 ) Meaning endsWith startsWith valueOf trim substring compareTo Method
    12. 12. Pattern Example <ul><li>Suppose students are assigned a three-digit code: </li></ul><ul><ul><li>The first digit represents the major (5 indicates computer science); </li></ul></ul><ul><ul><li>The second digit represents either in-state (1), out-of-state (2), or foreign (3); </li></ul></ul><ul><ul><li>The third digit indicates campus housing: </li></ul></ul><ul><ul><ul><li>On-campus dorms are numbered 1-7. </li></ul></ul></ul><ul><ul><ul><li>Students living off-campus are represented by the digit 8. </li></ul></ul></ul>The 3-digit pattern to represent computer science majors living on-campus is 5[123][1-7] first character is 5 second character is 1, 2, or 3 third character is any digit between 1 and 7
    13. 13. Regular Expressions <ul><li>The pattern is called a regular expression . </li></ul><ul><li>Rules </li></ul><ul><ul><li>The brackets [ ] represent choices </li></ul></ul><ul><ul><li>The asterisk symbol * means zero or more occurrences. </li></ul></ul><ul><ul><li>The plus symbol + means one or more occurrences. </li></ul></ul><ul><ul><li>The hat symbol ^ means negation. </li></ul></ul><ul><ul><li>The hyphen – means ranges. </li></ul></ul><ul><ul><li>The parentheses ( ) and the vertical bar | mean a range of choices for multiple characters. </li></ul></ul>
    14. 14. Regular Expression Examples Matches AZxx , CAxx , and COxx , where x is a single digit. (AZ|CA|CO)[0-9][0-9] Matches wad, weed, bad , and beed . [wb](ad|eed) A valid Java identifier consisting of alphanumeric characters, underscores, and dollar signs, with the first character being an alphabet. [a-zA-z][a-zA-Z0-9_$]* A single digit 0, 1, or 3. [013] A single character that is either a lowercase letter or a digit. [a-z0-9] A single digit that is 0, 1, 2, 3, 8, or 9. [0-9&&[^4567]] Any two-digit number from 00 to 99. [0-9][0-9] Description Expression
    15. 15. The replaceAll Method <ul><li>The replaceAll method replaces all occurrences of a substring that matches a given regular expression with a given replacement string. </li></ul>Replace all vowels with the symbol @ String originalText, modifiedText; originalText = ...; //assign string modifiedText = originalText.replaceAll ( &quot;[aeiou]&quot; , &quot;@&quot; ) ;
    16. 16. The Pattern and Matcher Classes <ul><li>The matches and replaceAll methods of the String class are shorthand for using the Pattern and Matcher classes from the java.util.regex package. </li></ul><ul><li>If str and regex are String objects, then </li></ul><ul><li>str.matches ( regex ) ; </li></ul><ul><li>is equivalent to </li></ul><ul><li>Pattern pattern = Pattern.compile ( regex ) ; </li></ul><ul><li>Matcher matcher = pattern.matcher ( str ) ; </li></ul><ul><li>matcher.matches () ; </li></ul>
    17. 17. The compile Method <ul><li>The compile method of the Pattern class converts the stated regular expression to an internal format to carry out the pattern-matching operation. </li></ul><ul><li>This conversion is carried out every time the matches method of the String class is executed, so it is more efficient to use the compile method when we search for the same pattern multiple times. </li></ul><ul><li>See the sample programs Ch9MatchJavaIdentifier2 and Ch9PMCountJava </li></ul>
    18. 18. The find Method <ul><li>The find method is another powerful method of the Matcher class. </li></ul><ul><ul><li>It searches for the next sequence in a string that matches the pattern, and returns true if the pattern is found. </li></ul></ul><ul><li>When a matcher finds a matching sequence of characters, we can query the location of the sequence by using the start and end methods. </li></ul><ul><li>See Ch9PMCountJava2 </li></ul>
    19. 19. The String Class is Immutable <ul><li>In Java a String object is immutable </li></ul><ul><ul><li>This means once a String object is created, it cannot be changed, such as replacing a character with another character or removing a character </li></ul></ul><ul><ul><li>The String methods we have used so far do not change the original string. They created a new string from the original. For example, substring creates a new string from a given string. </li></ul></ul><ul><li>The String class is defined in this manner for efficiency reason. </li></ul>
    20. 20. Effect of Immutability We can do this because String objects are immutable.
    21. 21. The StringBuffer Class <ul><li>In many string processing applications, we would like to change the contents of a string. In other words, we want it to be mutable. </li></ul><ul><li>Manipulating the content of a string, such as replacing a character, appending a string with another string, deleting a portion of a string, and so on, may be accomplished by using the StringBuffer class. </li></ul>
    22. 22. StringBuffer Example Changing a string Java to Diva StringBuffer word = new StringBuffer ( &quot;Java&quot; ) ; word.setCharAt ( 0, 'D' ) ; word.setCharAt ( 1, 'i' ) ; word : StringBuffer Java Before word : StringBuffer Diva After
    23. 23. Sample Processing <ul><li>Replace all vowels in the sentence with ‘X’. </li></ul>char letter; String inSentence = JOptionPane.showInputDialog ( null , &quot;Sentence:&quot; ) ; StringBuffer tempStringBuffer = new StringBuffer ( inSentence ) ; int numberOfCharacters = tempStringBuffer.length () ; for ( int index = 0; index < numberOfCharacters; index++ ) { letter = tempStringBuffer.charAt ( index ) ; if ( letter == 'a' || letter == 'A' || letter == 'e' || letter == 'E' || letter == 'i' || letter == 'I' || letter == 'o' || letter == 'O' || letter == 'u' || letter == 'U' ) { tempStringBuffer.setCharAt ( index, 'X' ) ; } } JOptionPane.showMessageDialog ( null , tempStringBuffer ) ;
    24. 24. The append and insert Methods <ul><li>We use the append method to append a String or StringBuffer object to the end of a StringBuffer object. </li></ul><ul><ul><li>The method can also take an argument of the primitive data type. </li></ul></ul><ul><ul><li>Any primitive data type argument is converted to a string before it is appended to a StringBuffer object. </li></ul></ul><ul><li>We can insert a string at a specified position by using the insert method. </li></ul>
    25. 25. The StringBuilder Class <ul><li>This class is new to Java 5.0 (SDK 1.5) </li></ul><ul><li>The class is added to the newest version of Java to improve the performance of the StringBuffer class. </li></ul><ul><li>StringBuffer and StringBuilder support exactly the same set of methods, so they are interchangeable. </li></ul><ul><li>There are advanced cases where we must use StringBuffer, but all sample applications in the book, StringBuilder can be used. </li></ul><ul><li>Since the performance is not our main concern and that the StringBuffer class is usable for all versions of Java, we will use StringBuffer only in this book. </li></ul>
    26. 26. Problem Statement <ul><li>Problem statement: </li></ul><ul><li>Write an application that will build a word concordance of a document. The output from the application is an alphabetical list of all words in the given document and the number of times they occur in the document. The documents are a text file (contents of the file are an ASCII characters) and the output of the program is saved as an ASCII file also. </li></ul>
    27. 27. Overall Plan <ul><li>Tasks expressed in pseudocode: </li></ul>while ( the user wants to process another file ) { T ask 1: read the file ; Task 2: build the word list ; Task 3: save the word list to a file ; }
    28. 28. Design Document Another helper class for maintaining a word list. Details of this class can be found in Chapter 10. WordList Classes for pattern matching operations. Pattern/Matcher A helper class for opening a file and saving the result to a file. Details of this class can be found in Chapter 12. FileManager The key class of the program. An instance of this class managers other objects to build the word list. Ch9WordConcordance The instantiable main class of the program that implements the top-level program control. Ch9WordConcordanceMain Purpose Class
    29. 29. Class Relationships class we implement helper class given to us FileManger Ch9Word Concordance Matcher WordList Pattern Ch9Word ConcordanceMain (main class)
    30. 30. Development Steps <ul><li>We will develop this program in four steps: </li></ul><ul><ul><li>Start with a program skeleton. Define the main class with data members. Begin with a rudimentary Ch9WordConcordance class. </li></ul></ul><ul><ul><li>Add code to open a file and save the result. Extend the existing classes as necessary. </li></ul></ul><ul><ul><li>Complete the implemention of the Ch9WordConcordance class. </li></ul></ul><ul><ul><li>Finalize the code by removing temporary statements and tying up loose ends. </li></ul></ul>
    31. 31. Step 1 Design <ul><li>Define the skeleton main class </li></ul><ul><li>Define the skeleton Ch9WordConcordance class that has only an empty zero-argument constructor </li></ul>
    32. 32. Step 1 Code <ul><ul><ul><li>Directory: Chapter9/Step1 </li></ul></ul></ul><ul><ul><ul><li>Source Files: </li></ul></ul></ul><ul><ul><ul><li> </li></ul></ul></ul>Program source file is too big to list here. From now on, we ask you to view the source files using your Java IDE.
    33. 33. Step 1 Test <ul><li>The purpose of Step 1 testing is to verify that the constructor is executed correctly and the repetition control in the start method works as expected. </li></ul>
    34. 34. Step 2 Design <ul><li>Design and implement the code to open and save a file </li></ul><ul><li>The actual tasks are done by the FileManager class, so our objective in this step is to find out the correct usage of the FileManager helper class. </li></ul><ul><li>The FileManager class has two key methods: openFile and saveFile . </li></ul>
    35. 35. Step 2 Code <ul><ul><ul><li>Directory: Chapter9/Step2 </li></ul></ul></ul><ul><ul><ul><li>Source Files: </li></ul></ul></ul><ul><ul><ul><li> </li></ul></ul></ul>
    36. 36. Step 2 Test <ul><li>The Step2 directory contains several sample input files. We will open them and verify the file contents are read correctly by checking the temporary echo print output to System.out. </li></ul><ul><li>To verify the output routine, we save to the output (the temporary output created by the build method of Ch9WordConcordance) and verify its content. </li></ul><ul><li>Since the output is a textfile, we can use any word processor or text editor to view its contents. </li></ul>
    37. 37. Step 3 Design <ul><li>Complete the build method of Ch9WordConcordance class. </li></ul><ul><li>We will use the second helper class WordList here, so we need to find out the details of this helper class. </li></ul><ul><li>The key method of the WordList class is the add method that inserts a given word into a word list. </li></ul>
    38. 38. Step 3 Code <ul><ul><ul><li>Directory: Chapter9/Step3 </li></ul></ul></ul><ul><ul><ul><li>Source Files: </li></ul></ul></ul><ul><ul><ul><li> </li></ul></ul></ul>
    39. 39. Step 3 Test <ul><li>We run the program against varying types of input textfiles. </li></ul><ul><ul><li>We can use a long document such as the term paper for the last term’s economy class (don’t forget to save it as a textfile before testing). </li></ul></ul><ul><ul><li>We should also use some specially created files for testing purposes. One file may contain one word repeated 7 times, for example. Another file may contain no words at all. </li></ul></ul>
    40. 40. Step 4: Finalize <ul><li>Possible Extensions </li></ul><ul><ul><li>One is an integrated user interface where the end user can view both the input document files and the output word list files. </li></ul></ul><ul><ul><li>Another is the generation of different types of list. In the sample development, we count the number of occurences of each word. Instead, we can generate a list of positions where each word appears in the document. </li></ul></ul>